We're in the midst of assembling the massive primary mirror of the James Webb Space Telescope (which is comprised of 18 gold-coated segments) at NASA's Goddard Space Flight Center in Greenbelt, Maryland. JWST is an engineering challenge, and when complete, this cutting-edge space telescope will be a giant leap forward in our quest to understand the Universe and our origins. It will examine every phase of cosmic history: from the first luminous glows after the Big Bang; to the formation of galaxies, stars, and planets; to the evolution of our own solar system. As the Optical Telescope Element Manager, I would be happy to answer questions about the construction of this telescope. For more information, visit our website

I will be back at 2 pm EST(11 am PST, 7 pm UTC) to answer your questions, ask me anything!

ETA: It's nearly 3:15 and Lee has to run - thank you all for your questions!

Unlike HST, JWST is highly sensitive to infrared wavelengths ranging from about 1.7um’s to 28 um’s which opens the door to many, many things but here are three that really excite me:

JWST can see the very first stars and galaxies forming which were created when the universe was about 250 million years old. As the universe has expanded, this light has been stretched from visible light from stars and galaxies into near infrared wavelength sweetspot of JWST. The older the objects, the more the light is stretched into the infrared. This was the main reason for building Webb the way it is!! HST is sensitive at ultraviolet and visible wavelengths but loses sensitivity at 1.7 um’s (and has none at all above 2.5um’s) so it can only see “older” objects when the universe was already about a billion years old and galaxies are more developed. The HST deep fields are incredible, but we want to look farther back in time and figured out how the galaxies formed and see those first objects.

JWST’s infrared capabilities allow us to study gas in exoplanets. Using a technique called transits, one can compare the spectrum of a star hosting an exoplanet when that exoplanet passes in front of the star and then behind it. The comparison tells you what light is being absorbed. The infrared signature will indicate what gas types are there - and the infrared is ideal for seeing signatures of all sorts of interesting molecules including those that are typical of life (like methane, CO2, etc). HST can see some molecules but is not nearly as sensitive (due to being smaller) and its shorter wavelength limits its ability to see really important ones. It’s doubtful HST will find life (unless we are really lucky), but it will tell us a lot about planets that orbit other stars.

Not only can JWST see exoplanet atmospheres, but it can also study the planets in our solar system providing spectral information on the gases and how they change and also take amazing pictures. JWST can actually create thermal maps of the planets and also look at comets and moving objects.

In theory, there should be no difference between practice and theory. In practice, there often is. As we get more data, we find the gaps in our theories, gaps that can cause theories to be completely re-written. Dark energy was postulated after more data was gathered on the rotation of galaxies; relativity was confirmed by very fine measurements that showed the location of Mercury was not where Newtonian physics said it would be due to the gravitational lensing of the sun.

There are still a lot of "what if's" in early universe cosmology. The further we can look back, the more of those we can answer.

The fact is that we haven't had telescopes powerful enough to see the first stars and galaxies forming in the early universe! Yes, there are theoretical predictions about the first stars (ie, that they were 30 to 300 times as massive as our Sun and millions of times as bright, burning for only a few million years before exploding as supernovae). But we don't have observations of these things. And we don't know exactly when these first stars formed, and when the reionization process started to occur (the point when most of the neutral hydrogen was reionized by the increasing radiation from the first massive stars). Understanding the first stars is really critical since they greatly influenced the formation of later objects like galaxies. These first bright objects are like seeds for the later formation of larger objects.

I'm curious about some of the mechanics involved in the JWST. The mirrors, as you said, are massive. How do you ensure not only that the mirrors don't potentially bump and scratch each other during ascent, but then that they are properly aligned and collimated once they are deployed?

Since the JWST will be orbiting the L2 Lagrange point, there can be no service missions (manned, anyway). Everything has to go right the first time. What lessons were learned from Hubble (regarding optical testing or otherwise), and how has that affected work on the JWST?

Great questions! We actually launch the mirrors retracted and then deploy them almost a half inch once in orbit. We actually have special collision avoidance algorithms to make sure the mirrors don't run into each other, ever, including on the ground.

For testing, we have two big differences from HST. First, since we are an active telescope, we can actually somewhat shape the primary mirror in space but how we set our actuators (which are the motors on the back of mirrors that move them around). We could actually correct a Hubble like error that way. Also, we do end to end testing of the full telescope which was not done on HST.

I would like to piggyback onto your question, and ask - even though there "cannot" be any service missions, is the JWST designed in a way that it "could" be serviced in the future, if the need should arise, and the money & means were to be provided?

In the early days of the Webb project, studies were conducted to evaluate the benefits, practicality and cost of servicing Webb either by human space flight, by robotic missions, or by some combination such as retrieval to low-Earth orbit. Those studies concluded that the potential benefits of servicing do not offset the increases in mission complexity, mass and cost that would be required to make Webb serviceable, or to conduct the servicing mission itself.

No, they didn't design serviceability into JWST despite Constellation talking up their ability to go to L2. Even to this day the Orion program's planning a mission to a near Earth asteroid. Going to L2 is roughly equivalent.

It's probable that JWST opted against serviceability to keep costs down, and also because a design that depends on being serviced puts them at the mercy of NASA's currently nonexistent human spaceflight capability.

Will the spectrometers on JWST be sensitive enough to detect biosignatures (eg O2) in the atmospheres of nearby, confirmed exoplanets (if such a candidate planet is found)? If so, can you give any estimate on what conditions would be necessary for such a detection to be made by JWST?

Second: JWST is mainly being sold as as an instrument to study the distant universe. However, it will also help us understand our own solar system as well. What do you think JWST's biggest contribution to solar system and planetary science will be?

JWST will be able to detect biosignatures of super-earth and larger planets (not quite earth size). However, JWST's infrared wavelengths allow us to detect the signatures of all sorts of interesting molecules like Methane, Co2, water. We can point JWST at the planets in our solar system and comets for both imaging and to analyze their gaseous compositions.

Dr. Mark Clampin, the James Webb Space Telescope observatory project scientist, did a AMA tweet session about exoplanets. I thought it was interesting, and so have pasted the whole thing here. I've taken the liberty to edit it for format.

Q: How soon can we see photos of some exoplanets?

A: JWST will be able to image young gas giant exoplanets with several of its instruments.

Q: Will the possible loss of the Kepler spacecraft have a significant effect on the JWST exoplanet mission?

A: [Kepler] has successfully found thousands of transiting candidates for us to study.

Q: What other info can be gathered through spectrography about the 3 new exoplanet candidates orbiting Gliese 667C?

A: Spectroscopy can tell us atmospheric composition.

Q: Spectroscopy on exoplanet atmospheres is really exciting, will they be able to do that on fairly thin atmospheres (<1 atm)?

Science operations start approximately 6 months after launch. (We need time post-launch to align the mirrors, calibrate the instruments, etc.) How do we choose what to observe once science operations start? Through a pretty standard proposal process (the same way Hubble and other observatories work) - scientists propose for observing time and a panel of peers decides which proposals get time on the telescope.

JWST will be able to image things like exoplanets, but they're really going to be pixels - they won't be like Voyager's images of our own solar system. Where JWST will really shine is going to be spectroscopy of exoplanet atmospheres. Characterizing their atmosphere is going to be really telling. And we're of course looking for planets with atmosphere's similar to Earth!

First let me just say that I am PUMPED that you decided to do this AMA!

In terms of functionality, how many different physical filters will there be? What wavelength ranges do they cover?

After detectors collect the light and have the information stored in the memory, is there any further filtering effects that can be applied through software to show certain phenomena?

I know that physical filters aren't enough to see xrays/gamma rays so certain detectors are needed. What detectors will be on-board the JWSP? What will they be used for?

Hardware-wise, what communication protocol are you using to communicate among on-board devices? Is there a Master with multiple slaves or is the hierarchy more evenly distributed? To communicate with Earth are you using an OSI based protocol or is there one more customized?

Software-wise, other than embedded C, is there any other language that is dominant?

We won't be detecting x-rays or gamma-rays - just infrared light and a little bit of optical.

JWST will use two types of detectors: four mega-pixel near infrared (NIR) mercury-cadmium-telluride detectors for wavelengths 0.6-5 microns, and one mega-pixel mid-IR silicon-arsenic detectors for 5-29 microns.

The biggest challenge was that there was multiple challenges. We had to fit a 6.5m diameter telescope in a rocket that is smaller than that. We needed our primary mirror 10 times lighter than Hubble's primary mirror. And all of this has to be built at room temperature but operate at 50 Kelvin which is incredibly cold.

Communication is the number one thing. This project is international so we have to communicate in how we schedule, budget, technical interfaces, specifications. You can never have too much communication!!

I imagine once the mirror is installed it is checked over thoroughly for any possible damage that might have been done to it during installation. My question is, if any is detected, would the mirror have to be completely removed to be repaired, considering the the specialized equipment that was needed to fabricate it, or is there some method to spot-fix it?

After the mirrors are physically installed, we have to hook up the cables and check out the electrical interfaces. As part of that, we actually deploy the mirrors just a little bit. We have covers on the mirrors that protect them and continue to monitor mirrors as we install them.

This is a great question. These mirrors are so light, gravity actually deforms them. So we have to use modeling to predict the gravitational deformations and account for that in all of our testing. We ground those models in testing. This is one of the engineering feats of JWST that will serve as a stepping stone to future larger telescopes.
Lee

Right now, we have covers on the mirrors whenever the mirrors are face up, like right now. Check out this hot off the press image of the first 6 mirrors on the telescope with black covers on our shiny gold coatings.

I have a little more info for you on the mirror covers from Paul Geithner, our deputy project scientist (technical).

He says: We will take them off for a center of curvature test at Goddard and actually they are off anytime the telescope is not "cup-up" because they merely rest on the mirror edges and aren't firmly attached so they are not used when the mirrors are not looking up. This goes for when they will be at JSC and at Northrop, when not cup-up. When they are not on and we can guard against contamination without interfering with something, we will have the telescope under a cover of some kind (such as during the big vibration test next year).

The covers will not be on for launch - the mirrors will be uncovered at some point in 2018 while at Northrop for final observatory integration and test.

I am so glad that the JWST is going to make it to space. I was worried that they were going to pull the funding on it awhile back. I think that it is going to be worth every bit of money it costs, as I have no doubt there are a legion of scientists working diligently to see it through to completion.

Anyways, my question is, how are micrometeor strikes being planned for? With so many mirrors exposed out into space, is there a certain amount of strikes it can tolerate per mirror?

Lastly, if you could comment on the two telescopes that were gifted to NASA by the guys in black suits. Do you think that NASA will be able to carve out some funds to utilize the hardware?

We actually have predictions for how many micrometeoroids of various sizes will hit the telescope. The sunshield is actually designed to allow small penetrations which is one reason it has so many layers (five). For mirrors, we actually tested the mirrors at Marshall Space Flight Center about a decade ago to show that super fast particles the size of micrometeoroids only make small indentations. We account for those effects in our performance predictions. - Lee

Hello Mr Lee, I have a question on spectrography. Recently Wolf 1061c was deemed to be potentially habitable. Considering that it is positioned "only" 14 light years away, it is possible to take spectrograph of its atmosphere using the James Webb Telescope? Thank you for taking your time to do this!

I asked Mark Clampin, one of our scientists, about this for you. He says: "It might be possible if Wolf1061c turns out to be a transiting exoplanet. The authors of the discovery paper calculate a ~6% probability that Wolf106c may be a transiting planet."

Yes it is. There is something called the Nyquist limit which is the finest level of detail that one can resolve. It depends on the wavelength and diameter of the primary mirror in our case. So there is a limit to what JWST can resolve based on the diameter.

Hi Lee, Thanks for the AMA! I wrote a proposal earlier this year for segmented mirror development for balloon borne telescopes. Actuators for the actively controlled segments are obviously one of the key design elements for such a system. I tried to find some publications on the capabilities and specs of the JWST actuators but came up mostly empty handed. Do you have any good references on the subject?

The JWST will be the most complicated telescope launch in history with countless unpacking steps once it leaves Earth. I'm curious as to what might happen if it doesn't unpack properly. Can we send someone or something to fix it? Do we construct a new one?

So, uhm, 11 days have gone by, so i'll take this one for the sake of knowledge. For what i have read in this AMA, the JWST will not be serviceable as the Hubble was, Hubble was launched into a low-Earth orbit, easy and safely reachable by the manned shuttles. JWST will be launched way farther, Langrangian point 2, outside Earth. Rather harsh environment for a human mission. They are making everything possible to make a flawless launch and deployment, any issue would be ignored and/or skipped by doing appropriate calculations.

The secondary mirror was actually the hardest mirror to make because it is convex. It's actually bigger than the Spitzer space telescope primary mirror which tells you just how big Webb is compared to the largest infrared telescope in space.

Engineering is all about optimization...if you call that compromise.

Three mirror anastigmats have larger fields of view.

I was hired by NASA to help fix the Hubble Space Telescope when it has the spherical aberration problem. After 10 years on Hubble and 3 years developing instrument technology, I got hired to be OTE manager for Webb.

You need each mirror aligned to a fraction of a wavelength of light. We use a series of image based algorithms we call Wavefront Sensing and Control.

Could you elaborate?

The secondary mirror was actually the hardest mirror to make because it is convex. It's actually bigger than the Spitzer space telescope primary mirror which tells you just how big Webb is compared to the largest infrared telescope in space.

Imagine you have a large 6.5 meter diameter parabolic surface. Now imagine you cookie cutter out hexagonal pieces that are each about 1.3 meters diameter. That is pretty much the JWST primary mirror. So each piece is an off axis section of a parabola. But the cool thing is the prescription of those mirrors repeats itself and there are only 3 unique prescriptions. So we have 6 of each prescription type. If you are good in computer modeling, you can check this out!

There were 10 different technologies developed for JWST. For the telescope alone, this included new technology lightweight cryogenic (cold) mirrors, wavefront sensing and control algorithms which let us align the mirrors in space to a 1/10,000the the diameter of a human hair, and stable composites that operate at -400 degrees Fahrenheit. We've had to figure out how to test the telescope end to end optically at -400F. We've had to figure out how to assemble the system robotically and assure every mirror can be aligned in space. So there are just a few of the innovations.

Gee, I wish we could get some close-up views of how these delicate and intricate mirror segments are being handled by the robotic arm. I have only been able to see the top view, but not how they are supported from below much less all the hardware and electronics connections to the backplane. Any way we could get some? Thanks.

It took almost a decade of planning and practicing to get to this point. The incredible teams from Harris Corporation and NGAS and Ball Aerospace and Goddard Space Flight Center have thought about every little detail...

Hi, Lee, thanks for being here! Sorry if this is off-topic, but I want to ask about an earth-based telescope.

As I'm sure you're aware, the State Supreme Court of Hawaii recently rescinded the building permit for the construction of the Thirty Meter Telescope on Mauna Kea. That was a small victory for a movement born out of the religious views of native Hawaiians who view Mauna Kea as a sacred site. As far as the pro-TMT opinions I have read are concerned, this is nothing more than a delay in the eventual construction of the TMT, but it is a costly delay nonetheless.

What impact do you think this decision have on the construction of the next generations of telescopes?

Is there anything that Reddit users can do to facilitate the construction or improve public perception of the next generation of telescopes?

We worry about several sources of stray light and control them through internal design features like baffles and model it all. We worry about sun, earth and moon. We also worry about Zodiacal light and bright stars. This is one of the major innovations of JWST. It's surprising that you don't need a tube but when you are at L2 and you have a big sunshade to protect you from sun, earth and moon, you can control these things.

How long is it expected to stay operational and what's the most likely cause of its eventual failure, and what happens to it when it stops? For example limited station keeping fuel vs. mechanical failures vs. electronics/sensor failure, what will go first?

JWST has a 5 year life requirement and a 10 year goal and we expect it to last longer. Fuel is the most likely life limiting item although maybe clever people could find a way to refuel JWST someday if needed.

By the way, L2 is pretty benign environment compared to low earth orbit which means once we are up there working, we should last a while.

What Keeps you up at night as this gets closer to launch? I'm sure you guys have thought of everything but what's that one thing you can do little about? A random gamma ray burst frying the electronics? A high speed particle impacting the mirror?

Thanks for coming by Mr. Feinberg! It is no exaggeration to say that I find the project you are working on to be the single most exciting thing in the entire world right now.

The Hubble has given us a great wealth of iconic images of our universe. Which famous structures/systems that have been captured by the Hubble are you most interested to revisit through the lens of the JWST?

What technical hurdle was the most challenging to overcome in the design and construction of the JWST to date?

Lastly, a bit off topic, but I see from your profile on NASA's site that you are also a pianist, so I would love to know which piece is your favorite to perform.

Can you give live updates with pictures of it being installed?
For questions, how much does the entire device cost?
How long has it taken to create from the moment it was approved?
What part is the most dangerous/risky in terms of things going wrong?

Back in late 80's, as a young boy I used to collect all of the newspaper articles about the Hubble space telescope and what it might be able to do.

Almost 30 years later, I recently saw the Imax video about Hubble with my son at the Kennedy space centre. I could never have dreamed of what it uncovered. It is just amazing to me what this instrument has been able to see. It's hard for a man to grasp what it means to see back 10 billion years.

JWST will see the universe when it was approximately 300 million years old and when the very first stars and galaxies formed. What's cool about JWST is that is not only an amazing telescope but also a path to future telescopes that are even bigger. Perhaps 30 years from now a 12-20 meter class segmented telescope will be built that can do even more incredible science including a statistical survey looking for signatures of life on earth-like Exoplanets...
Lee

Why does it cost so much, or more importantly, why does the James Webb (who was a budget hawk) Space Telescope cost so much more than originally budgeted?

How much of the cost and complexity was due to mass and size constraints of rocket payload capabilities? There are some larger heavier rockets on the horizon. If their immense fairings were available, would JWST have been easier/cheaper to design and build?

Has there been any thought given to using the JWST for purposes outside its exoplanet mission? For example, could it be trained on planets inside our solar system to get higher-resolution images than have been available before? Could we use it to (let's say) create more detailed maps of the surface of Jupiter's moons, Pluto, etc.?

JWST will be used but thousands of astronomers all over the world. They will come up with proposals which will be reviewed and the best ones will get time on the telescope. This is actually how Hubble works. JWST will be used to study nearly stars, galaxies, early universe, black holes, you name it!
Lee

the JWST will be the furthest from earth and biggest space telescope. what do you think could be the next step? are inflatable mirrors possible and is it realistic to put a telescope in gravitational lense distance from the sun?

How much of the total cost of JWT is development vs build (and launch)? If we lost JWT on launch, or there was a machining error on an important servo, crippling it's functionality, how much would it cost to put up a 2nd JWT?

Follow up: given the demand for telescope time, why don't we put up multiple JWTs?

Money and politics. According to a 1997 poll, Americans estimate NASA's budget to be ~20% of the federal budget, when in fact it is a fraction of a percent (and this number is steadily decreasing).

Projects like HST and JWST are expensive. When roughly one-fifth of the country believes that the earth is 6000 years old, it's rather difficult to convince politicians to "waste" money on things like JWST.

JWST wasn't designed to be serviced. There are a lot of historical reasons for this. However, there are future telescopes in planning that will go to L2 and which people hope can be serviced. Whether you would do it with robots or people is the question...I hope it happens!
Lee

Hi, my dream is to be a physicist and I will be starting my degree next year, I just wanted to know if you had any tips on being a scientist. for example, what do I say when I'm around professionals? and how would I get into a job like yours, do I need an amazing degree or loads of experience or both?
Thanks for answering the questions

Not OP. But I'm working on my Ph.D in physics right now. I'm assuming you mean starting your bachelor's next year. If so, don't worry too much about professionalism. It'll become natural as time goes on. Just try your best to be polite while also being friendly. Also accept criticism and don't be scared to ask "stupid" questions.

First, as a fellow Marylander I just have to say Hi! and thanks for the job creation!

(Warning Redditors, boring questions in coming!)

I'm curious about the precision necessary to build such a complex instrument. It seems to me that the margin for error would have to be enormously small to ensure that all components serve their purpose. In general I was wondering if you could riff on what quality control is like on such an important project, but that might be a little vague so I'll offer some more specific questions:

What is the best process currently for making precision parts for, well, almost anything at NASA?

In the recent past our scientists didn't have access to computer modeling, water jet or laser cutting, or CNC machines. How would the James Webb have been modeled and manufactured in the past?

How are individual components tested/checked for quality?

Are there any components that haven't changed over the decades, or could be found in older telescopes?

What is done with the unused parts, like the Spacely Sprocket that's .01 microns too small?

What, if any, safety precautions are put in place to protect these components from space debris, micro meteorites, or little grey men?

Obviously I'm joking about the little grey men. There are no little grey men. Are there little grey men?

Thank you for taking the time to do this AMA! Got my fingers crossed for an excellent send off!

Good afternoon, Lee! I was wondering how I could get access to the GSFC campus to see the JWST and the mirror installation. I've been on campus before amd wandered into building 29 before to check it out but nothing was going on. Also walked to building 7 amd got some mission stickers amd pins. Love the work you all are doing!!!

ImJohnathan - Thank you, we're so proud of our scientists, engineers & everyone who works here at NASA Goddard. We organize tours through the Office of Communications and through the Goddard Visitor's Center, give them a call and go from there. - Aries Keck, Goddard Social Media Team Lead

I've read in the FAQs page that the JWST will be able to look to galaxies almost right after the Big Bang, from about 100 to 250 million years after it. Will we ever be able to make a telescope able to look further back in time and farther in space, for example, 10 to 1 million years since the Big Bang? Or even directly look at the Big Bang itself?

Another thing that came in my mind is if there's any risk of collision with a Trojan, or other object?

Also, how long does it take to coat one mirror?

Finally, will the Hubble still operate after JWST's launch?

I feel fortunate for being able to ask someone working in the project! Thanks in advance!

We'll be looking for the first galaxies to form after the Big Bang, somewhere around a quarter of a billion years after it.

But we have seen further back than that already - the COBE and WMAP satellites have seen the residual heat signature leftover from the Big Bang about 380,000 years afterwards. This was, however long before the first bright objects like stars and galaxies were formed. You can read about this here: http://jwst.nasa.gov/firstlight.html

Yes, Hubble will still operate after JWST's launch - we are a scientific successor, not a replacement and their data will be complementary to each other. We will keep Hubble running as long as we possibly can.

If you're asking about the detectors, we'll use two types of detectors: four mega-pixel near infrared (NIR) mercury-cadmium-telluride detectors for wavelengths 0.6-5 microns, and one mega-pixel mid-IR silicon-arsenic detectors for 5-29 microns.

JWST will be able to observe our solar system planets and moons. What will the resolution be like? Will JWST be able to do groundbreaking research on these bodies that the spacecraft we have sent weren't able to do?